Management has the right to add duties to your job. However, if those duties are ones performed by positions in a higher classification and do not fall within your own classification specifications, you may be entitled to  work-out-of-class pay or a reclassification. If the duties being added to your job are within your classification (even if they are not currently in your job description), you are probably not entitled to more pay (unless they qualify for a differential). If you are concerned that you will not be able to perform all the additional work given to you, you should be sure to ask your supervisor to prioritize your work. Your supervisor should respond in a timely manner.